Isochronous data can generally be described as data which is non-packetized and of indeterminate, potentially continuous duration. Examples of isochronous data sources include video cameras, which output a substantially continuous stream of data representing images and associated sounds, and telephones, which output a substantially continuous stream of voice data. An example of an isochronous data sink is a video monitor which receives a substantially continuous stream of video data for display.
FIG. 1A schematically depicts isochronous data transfer. The data transfer is first initiated, such as by initiating a telephone conversation or beginning a video camera transmission 12. After initiating the data transfer, transmission of the data is provided substantially continuously for an indeterminate period, until termination of the telephone conversation or video transmission 14. Every bit transferred need not necessarily represent a data bit. "Housekeeping" bits to control destination and timing may be also transferred. Furthermore, the data being transferred may comprise "Null" data such as silence during a telephone conversation or transfer of a blank video image. One type of isochronous data transfer is the Fiber Distributed Data Interface-II (FDDI-II) as described, for example, in FDDI-II Hybrid Multiplexer, Revision 2.4, dated Mar. 25, 1991.
The increasing availability of multi-media computers and work stations that include isochronous data sources and sinks in addition to non-isochronous sources and sinks has increased interest in the transfer of isochronous data in a network environment. Many existing networks use non-isochronous data communications between stations on the network. Commonly used data transfer protocols include packet transfer systems and token ring systems.
An example of packetized data transfer is the commonly-used ethernet system. One implementation known as 10BASE-T is described in the draft Nine supplement to IEEE standard 802.3, dated Nov. 15, 1989. FIG. 1B shows a packet transfer 22.
In a token ring system, a node transfers data only upon receipt of an electronic token. One commonly used token ring system is described in IEEE standard 802.5 FIG. 1C shows a data transfer 23 in a token ring system.
Many previous attempts to accommodate isochronous data on these existing data networks result in disadvantageous operating characteristics. In some systems, the bandwidth available for a given isochronous source or sink degrades in direct proportion to the total number of isochronous data sources and sinks transmitting and receiving on the network. Also, the presence of isochronous sources and sinks degrades the non-isochronous bandwidth. Furthermore, in any isochronous system employing bidirectional links, the link cable length introduces a skew between data transmitted from the node to the hub and data received from the node by the hub. These delays introduce undesirable jitter and can be disconcerting to users of video and telephonic data.
In addition, existing isochronous systems also provide little or no compatibility with previous networks. This incompatibility necessitates extensive replacement of hardware or software to accommodate both isochronous and non-isochronous traffic. Thus a multi-media PC having ethernet capabilities and a video camera cannot simultaneously utilize both the isochronous and non-isochronous source/sinks.